Conventionally, as a general forming method of a tooth part of an internal gear, a mechanical machining method using a cutting unit having a cutter is known. A forming method of a tooth part using a cutting unit is explained with reference to FIG. 9.
The cutting unit 101 shown in FIG. 9 is a cutting unit for conducting a cutting process to a preformed gear, which is cylindrical and which has a small opening at an end and a large opening at the other end. The cutting unit 101 consists of a cutting tool 109 and a holding tool 110. The cutting tool 109 consists of a cutter 102 having a cutting tooth part 103, and a holder 104 holding the cutter 102 and movable forward and backward with respect to the preformed gear. The holding tool 110 has a receiving jig 105 arranged opposite to the cutting tool 109, and a plurality of chuck members 108. The receiving jig 105 has a first receiving part 106 having an outside diameter that is substantially the same as an inside diameter of the large opening of the preformed gear, and a second receiving part 107 extending outward in the radial direction serially from the first receiving part 106.
The cutting unit 101 is used to form a tooth part of an internal gear. Concretely, at first, the large-opening side of the preformed gear is inserted into the first receiving part 106 of the receiving jig 105. Then, the end surface of the large opening is brought into contact with the second receiving part 107, so that the preformed gear is placed on the receiving jig 105. After that, the chuck members 108 hold the end surface of the small opening of the preformed gear. Then, the cutter 102 integrated with the holder 104 is moved toward the preformed gear, so that the cutter 102 is brought into contact with the inside surface of the preformed gear. Thus, the cutting process by means of the cutter 102 is started, and then the tooth part 153a is formed.
Herein, in general, for a gear, it is preferable to form a crowning on a tooth flank of a tooth part of the gear, in order to inhibit abnormal sound generation and to improve contact manner of the tooth flank.
However, in the cutter 102 of the cutting unit 101 as described above, it is difficult to control minute movement thereof. Thus, it is difficult to form a crowning on the tooth part 153a. Then, in order to form a crowning on the tooth part 153a of the internal gear, after the tooth part 153a is formed by the cutting unit 101, the tooth trace of the tooth part 153a is adjusted by another grinding process to form the crowning on the tooth flank.